A few startling studies in the past decade have indicated that up to three-quarters of Americans are deficient in vitamin D, and as a result, sales of the vitamin supplement increased tenfold from 2001 to 2009. But now, experts say that though vitamin D is important, it might not help prevent all the diseases it was once thought to prevent.

The Institute of Medicine, part of the National Academy of Sciences, reviewed more than 1,000 studies on vitamin D and determined that, though vitamin D does help the body absorb calcium and strengthen bones, tests to determine whether it prevents cancer, heart disease or a host of other conditions are, so far, inconclusive. (Source: http://bit.ly/fkr0Py)

The institute also noted that there are disparities in vitamin D screening methods and in the recommended amount of vitamin D doctors say a person needs. People can absorb vitamin D from being in sunlight, or from eating fish oil or coldwater fish, like salmon or tuna. Milk and other dairy products are sometimes fortified with vitamin D.

Here are the latest numbers on intake recommendations and who's really at risk:

20

Nanograms of vitamin D a healthy person should have per milliliter of blood, according to the Institute of Medicine's 2010 report. Some earlier studies used 30 nanograms as the threshold.

600

International units (IU) the institute recommends as the daily dose of vitamin D for most children and adults. Those over age 70 should increase to 800 IU. The measure typically can be found on the labels that come with the vitamin.

39%

Patients with bone fractures at the University of Missouri trauma center who had vitamin D levels below 20 nanograms per milliliter of blood, according to a 2012 report. Although the Institute of Medicine does not recommend vitamin D screenings for all adults, it suggests screenings for people with other risk factors, including osteoporosis, celiac disease or other conditions that make it hard to absorb nutrients.